The present invention relates to a furnace particularly for heat treatments of glass sheets.
It is known that a wide variety of glass sheets is currently commercially available, the mechanical characteristics whereof are often strongly dependent on heat treatments performed after producing said sheets.
In particular, one of the most important heat treatments applied to glass sheets is heat tempering.
Heat tempering facilities currently include a furnace constituted by a longitudinally-elongated chamber in which a roller conveyor conveys the glass sheets, which are heated to a temperature which is, by way of indication, between 620 and 700xc2x0 C.
Two methods are currently used to heat said sheets: heat transmission by irradiation or heat transmission by convection.
Furnaces are currently commercially available which use one or the other of these heating methods.
However, there are also more advanced and complex furnaces which use both methods but do so alternately.
The means for irradiation heat transmission are currently substantially constituted by electrical resistors of the rod type with a ceramic core, by coiled electrical resistors which are suspended or contained in channels formed in the refractory material of the furnace, by electrical resistors of the panel type, or radiating panels supplied by gas-fired burners.
In furnaces that use heat transmission by forced convection, the air inside the chamber is instead channeled and recirculated by means of fans towards nozzles which project it onto the glass sheet surfaces.
The air can be heated by the electrical resistors located in front of the fans or by gas-fired burners, which heat it inside channeling ducts.
Although they are widely used, the above-described systems all have a common drawback, i.e., they are unable to uniformly heat the two opposite surfaces of the sheet being processed in order to avoid distortions thereof caused by temperature differences due indeed to the different heating conditions.
More specifically, the different heating conditions of the two surfaces arise from the different values of the overall heat transmission coefficients between said surfaces and the furnace environment.
These different boundary conditions in fact cause, in the transient thermal condition, an asymmetrical temperature distribution along the glass sheet cross-section.
The more conspicuous unevennesses furthermore occur when the glass sheet at room temperature (by way of indication, at 20xc2x0 C.) makes contact, inside the furnace, with the ceramic rollers, which are at a temperature of approximately 700xc2x0 C.
The heat transmission coefficient due to the roller contact heat resistance is much greater than the transmission coefficient due to the irradiation on the upper side of the sheet.
This entails a much greater temperature increase on the lower surface than on the upper surface.
Merely by way of indication, for glass temperature values below the limits at which relaxation effects occur (550xc2x0 C.), the temperature difference on the two surfaces entails a greater expansion of the lower surface, with a consequent warping of the sheet, which thus tends to become concave and touch the conveyor rollers only in the central part.
This highly negative effect produces, on the sheet surface, abrasions of different depths and conspicuousness which can cause the end product to be unacceptable.
This effect is also even more evident in the case of glass sheets coated on one surface with a low-emissivity coating.
Low-emissivity coatings in fact have the purpose of reducing heat transmission through the glass sheet.
Said sheets are thus coated in order to reflect infrared heat radiation (wavelengths between 2 and 20 micrometers), leading to a reduction in what is known as emissivity of the sheet surface.
In this manner, it is possible to obtain thermally insulating glass sheets with heat transmittance rates comparable with those of opaque portions.
When it is necessary to temper these sheets, the above-mentioned problem of heating the treated surfaces becomes even more significant, since it is not convenient to arrange the coated surface downwards, since it would make contact with the rollers and would deteriorate due to abrasion; however, if the coated surface is arranged upwards, the irradiation from the arch of the furnace is reflected and accordingly an undesirable unevenness in heating occurs which further increases the problem of roller contact.
U.S. Pat. No. 4,336,442 discloses a roller hearth furnace for heating glass sheets which includes a plurality of conveyor rollers arranged inside the furnace chamber for conveying glass sheets from an entrance to an exit of the chamber. Arranged above and below the conveyor rollers inside the furnace chamber are a plurality of combination radiation and convection heaters formed by electrically heated serpentine pipe structures inside of which air is fed from outside the furnace, which air is subsequently heated by the pipe structures and through nozzles provided on the pipe structures directed for providing blasts of hot convection currents to the glass sheets.
The aim of the present invention is to solve the above drawbacks of conventional commercially available furnaces, in particular by achieving high temperature uniformity on both surfaces of the glass sheet being treated, both when treating normal glass sheets and when treating sheets with surfaces coated with low-emissivity coatings.
Within the scope of this aim, an object of the present invention is to improve the quality of the product after treatment, avoiding the onset of heat-related stresses and the formation of surface abrasions.
Another object of the present invention is to provide a furnace in which it is possible to control and adjust the heating conditions of the sheets being treated.
Another object of the present invention is to provide a furnace which is particularly flexible from the operating point of view according to the type of sheet to be treated.
Another object of the present invention is to provide a furnace which has the advantages of convection furnaces and those of irradiation furnaces but does not have the corresponding drawbacks.
In accordance with the invention, there is provided a furnace particularly for heat treatments of glass sheets as defined in the appended claims.